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(No Rietveld link):
+is_valid_fd(int fd)
[...]
+ dummy_fd = dup(fd);
+ if (dummy_fd < 0)
+ return 0;
+ close(dummy_fd);
Why not use fstat() instead (does Windows have fstat()? And dup()?).
+ @unittest.skipIf(os.name == 'nt', "test needs POSIX semantics")
+ def test_no_stdin(self):
It would maybe be more direct with skipUnless(os.name == 'posix').
Finally, it's not that important, but it could maybe be possible to factorize the code, i.e. make a helper function that takes a list of streams and defines the preexec() function and code to test those streams, and then just call:
def test_no_stdin(self):
 out, err = self._test_with_closed_streams(['stdin'])
 [...]
def test_no_streams(self):
 out, err = self._test_with_closed_streams(['stdin', 'stdout', 'stderr'])
 [...]